This invention relates to a surface acoustic wave (SAW) oscillator comprising a plurality of parallel-connected surface acoustic wave resonators, each of which includes a pair of interdigitated electrodes mounted on a quartz substrate for propagating a surface acoustic wave.
The prior art surface acoustic wave oscillator comprises as shown in FIG. 1, an active element-containing circuit 2 which is connected to a surface acoustic wave oscillator 1 through the terminals 1a, 1b. With the conventional surface acoustic wave oscillator 1, a pair of interdigitated electrodes 4, 5 (formed of electrode elements 6, 7 respectively) are mounted on the main plane of a substrate 3 prepared from, for example, quartz for connection to said active element-containing circuit 2.
FIG. 2 shows an equivalent circuit of the surface acoustic wave oscillator of FIG. 1. According to this equivalent circuit, a resonator section 1 comprises a series circuit of an inductance L and capacitor C connected in parallel to another capacitor C.sub.T. A circuit section 2 including an active element consists of a series circuit of a capacitor C.sub.L and negative resistor -R. The resonance circuit section 1 and circuit section 2 are connected by terminals 1a, 1b.
The constants L, C, C.sub.T of the equivalent circuit can be freely selected by changing the number of the respective paired electrode elements or fingers of the interdigital electrodes, the thickness of said interdigital electrodes and the length of those portions of the respective paired electrode elements or fingers which are actually interdigitated with each other.
Where the resonance circuit section 1 of a surface acoustic wave oscillator represented by the above-mentioned equivalent circuit has an impedance Z, then said oscillator is oscillated at a frequency f satisfying the condition expressed by the equation (1) below: EQU 1/(j2.pi.fC.sub.L)+z=0 (1)
where j is imaginary symbol.
Referring to a single resonator, the impedance z thereof generally varies with ambient temperature. This means that the surface acoustic wave oscillator has a frequency largely governed by ambient temperature.
FIG. 3 is a curve diagram of the deviation .DELTA.f of the aforesaid oscillation frequency f of the oscillator relative to ambient temperature T, where the substrate thereof is formed of quartz and the main plane of the quartz is represented by the rotated Y cut plane. As apparent from FIG. 3, the frequency deviation .DELTA.f is substantially reduced to zero at the turnover temperature T.sub.p. Where, however, the turnover temperature T.sub.p is shifted either upward or downward, then the frequency deviation .DELTA.f increases along a second degree temperature coefficient. Compensation for the temperature dependency of oscillation frequency has hitherto been undertaken in a circuit including an active element. Yet said compensation has proved unsatisfactory.
A crystal oscillator which has been proposed to date to compensate for the temperature dependency of oscillation frequency includes the U.S. Pat. No. 3,821,666. According to this prior art, three bulk wave crystal vibrators are connected in parallel. This parallel circuit is connected to an active element-containing circuit. Said U.S. Patent oscillator is the temperature-compensation type which is intended to reduce frequency deviation over a prescribed temperature range by connecting in parallel the three crystal vibrators which collectively display a particular frequency deviation characteristic relative to ambient temperature.
The bulk vibrators included in a temperature compensation type oscillator present greater difficulties in manufacture, according as said oscillator is demanded to have a higher frequency. The reason is that a substrate of bulk wave crystal vibrator has to be made thinner in the inverse ratio to the increased frequency. Further, provision of, for example, wiring supports unavoidably give rise to variations in the properties of said temperature compensation type oscillator, whose practical application is therefore undesirably limited.
With a surface acoustic wave oscillator of this invention, the rotated Y cut plane of quartz is used as a substrate. A pair of interdigitated electrodes whose electrode elements are mounted on the main plane of the substrate in the alternately adjacent form are connected to an active-element containing circuit through connection terminals. According to the SAW resonator of this invention comprising a surface acoustic wave resonator presents a noticeable difference from the prior art bulk vibrator type oscillator in the properties resulting from the operation principle, construction and resonance condition, a plurality of surface acoustic wave resonators are connected in parallel and are so constructed that a prescribed relationship exists between the frequencies of the respective surface acoustic wave resonators as well as between the turnover temperatures. The technique of producing a surface acoustic wave oscillator embodying this invention has not been known to date, nor can be inferred from any prior art.